Range of possible values of pdev_num is 0-63. Use int8_t as data type.
That also fixes below warnings:

>> drivers/gpu/drm/amd/amdxcp/amdgpu_xcp_drv.c:59:58: warning: '%d'
>> directive output may be truncated writing between 1 and 11 bytes into
>> a region of size 9 [-Wformat-truncation=]
      59 |         snprintf(dev_name, sizeof(dev_name), "amdgpu_xcp_%d", pdev_num);
         |                                                          ^~
   drivers/gpu/drm/amd/amdxcp/amdgpu_xcp_drv.c:59:46: note: directive argument in the range [-2147483648, 63]
      59 |         snprintf(dev_name, sizeof(dev_name), "amdgpu_xcp_%d", pdev_num);
         |                                              ^~~~~~~~~~~~~~~
   drivers/gpu/drm/amd/amdxcp/amdgpu_xcp_drv.c:59:9: note: 'snprintf' output between 13 and 23 bytes into a destination of size 20
      59 |         snprintf(dev_name, sizeof(dev_name), "amdgpu_xcp_%d", pdev_num);
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 97d814fe ("drm/amd/amdxcp: Use unique name for partition dev")
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>